Three duplex modes, Frequency Division Duplex (FDD), Half-FDD (H-FDD) and Time Division Duplex (TDD), are currently supported by Long Term Evolution (LTE) systems.
FDD refers to uplink transmission and downlink transmission in different carrier frequencies to allow concurrent reception and transmission of signals by a base station and a User Equipment (UE); and TDD refers to uplink transmission and downlink transmission in the same carrier frequency to allow transmission and reception of signals respectively by a base station and a user equipment in different periods of time, or allow reception and transmission of signals respectively by a base station and a user equipment in different periods of time.
In wireless communication system of LTE and earlier wireless communication systems, a cell is configured with only one carrier, and the maximum bandwidth in LTE system is 20 MHz, particularly as illustrated in FIG. 1.
Compared with the peak rate of LTE system, the peak rate of Long Term Evolution-Advanced (LTE-A) system improves significantly, wherein the peak rate of LTE-A system is required to be 1 Gbps in the downlink and 500 Mbps in the uplink. The required peak rates can not be reached with only one carrier with a maximum bandwidth of 20 MHz. Thus, the bandwidth available to the user equipment in LTE-A system needs to be extended, and in view of this, Carrier Aggregation (CA) technology has been introduced, that is, a plurality of consecutive or inconsecutive carriers in the same base station (eNB) are aggregated together to serve the UE concurrently with a desirable rate. These carriers aggregated together are also referred to as Component Carriers (CCs). Each cell can be a component carrier, and cells (component carriers) in different eNBs can not be aggregated. In order to ensure the UE of the LTE system to be able to operate over each of the aggregated carriers, the bandwidth of each aggregated carrier is no more than 20 MHz, particularly as illustrated in FIG. 2. There are four carriers that can be aggregated in the base station of LTE-A system as illustrated in FIG. 2 so that the base station can transmit data with the user equipment over the four carriers concurrently to thereby improve the throughput of the system.
In the LTE system, the length of a radio frame is 10 ms and the length of a sub-frame is 1 ms in both the FDD mode and the TDD mode. Seven TDD uplink and downlink configurations are defined for a radio frame in each of the TDD modes, particularly as depicted in Table 1, where D represents a downlink (DL) sub-frame, U represents an uplink (UL) sub-frame, and S represents a special sub-frame of the TDD system.
TABLE 1(TDD uplink/downlink configurations)TDD uplink/Sub-frame indexdownlink configuration01234567890DSUUUDSUUU1DSUUDDSUUD2DSUDDDSUDD3DSUUUDDDDD4DSUUDDDDDD5DSUDDDDDDD6DSUUUDSUUD
In FDD mode of LTE system, a user equipment receives downlink data in sub-frame n−4 and feeds back signaling of whether the data in the downlink sub-frame needs to be retransmitted, that is, feeds back Acknowledgement/Negative Acknowledgement (ACK/NACK) information, also referred to as a Physical Downlink Shared Channel Hybrid Automatic Repeat ReQuest (PDSCH HARQ), in the uplink sub-frame n. When carriers are aggregated, ACK/NACK information corresponding to a plurality of downlink carriers in the sub-frame n−4 will be fed back in uplink sub-frame n in a primary carrier concurrently.
In TDD mode of LTE system, a UE may feed back, ACK/NACK information corresponding to a plurality of downlink sub-frames, in the same uplink sub-frame, that is, the UE detects Physical Downlink Shared Channel (PDSCH) transmission, or Physical Downlink Control Channel (PDCCH) indicating downlink semi-persistent scheduling release, in the downlink sub-frame n−k and feeds back corresponding ACK/NACK information in the uplink sub-frame n, wherein kεK, and values in the set K depend upon a TDD uplink and downlink configuration of the system and a particular sub-frame index, particularly as depicted in Table 2; and particularly special sub-frames of special sub-frame configuration 0 and 5 with a normal Cyclic Prefix (CP) and special sub-frames of special sub-frame configuration 0 and 4 with an extended CP have no ACK/NACK feedback, that is, the user equipment will not feed back ACK/NACK information for such special sub-frames.
TABLE 2(related TDD downlink K values: {k0, k1, . . . kM−1})uplink/downlinkSub-frame indexconfiguration01234567890——6—4——6—41——7, 64———7, 64—2——8, 7, 4, 6————8, 7, 4, 6——3——7, 6, 116, 55, 4—————4——12, 8, 7, 116, 5, 4, 7——————5——13, 12, 9, 8, 7, 5, 4, 11, 6———————6——775——77—
Table 2 reflects a PDSCH HARQ feedback timing indicated by a TDD uplink and downlink configuration. In LTE system, a plurality of radio frames are arranged in order, although Table 2 only takes a radio frame as an example and depicts values of K corresponding to respective uplink sub-frames, wherein n−k<0 indicates the last k-th downlink sub-frame in a preceding radio frame, for example, with n=2 and k=6, uplink sub-frame 2 in radio frame a carries ACK/NACK information corresponding to downlink sub-frame 6 in radio frame a−1.
However, there is not a solution to implement carrier aggregation across systems in the LTE system currently, and as a result, there is no HARQ feedback solution for downlink data when carrier aggregation is performed among TDD system and FDD system so far.